In the field of photosensitive imaging devices there is a desire for increased resolving capability, particularly where the imaging devices are incorporated in, e.g., surveillance systems or high-performance professional digital cameras. A practical benefit associated with increased resolution in surveillance systems is the ability to recognize a feature having relatively smaller dimensions in a given field of view. A practical benefit associated with increased resolution in high-performance professional digital cameras is the ability to enlarge photographs without obvious artifacts like pixilation.
One way of achieving improved resolution is to increase the number of picture elements (hereinafter “pixels”) used to sense a given field of view. The decision to increase pixels, however, is not cost-free. For example, if the overall dimensions of the sensor element are held constant, more pixels with less detecting area per pixel will be used to image a given field of view. A typical consequence of this is an increase in the susceptibility of the imaging system to artifacts associated with noise, e.g., the blotchy appearance of shadowy areas in images of scenes where in the actual view the shadowy areas were of relatively uniform illumination.
Another approach which can maintain a desired noise performance is to hold the individual pixel size constant, but to increase the overall dimensions of the imaging sensor. This also has negative consequences. In particular, such an approach will require fabrication in semi-conductor materials of larger imaging sensors typically accompanied by lower yields. Thus the imaging sensor will be more expensive. In addition, if the imaging sensor is used in combination with optics, e.g., a zoom lens, the optical elements comprising the zoom lens, and the overall size of the zoom lens will have to be larger to maintain the same field of view.
It is not surprising that given these constraints a hybrid approach is often pursued, i.e., the overall dimensions of the imaging sensor are increased while reducing somewhat the dimensions of the pixels. Nonetheless, even in compromise situations the same problems are encountered, e.g., susceptibility to noise and increased fabrication expense.
Problems are also encountered in the selection of imaging sensor technology, e.g., PIN, active PN or CCD, as each of these have their own respective problems as the size of the pixel comprising the imaging sensor decreases. For example, in active PN devices a portion of the pixel actually comprises non-photo-sensitive control circuitry. As the size of pixels decrease for a constant imaging sensor dimension, the control circuitry becomes a larger percentage of the device area, eventually to the point where noise effects become intolerable.
In environments where radiation events are possible, non-optical issues have to be taken into consideration. Different imaging sensor technologies have different radiation hardness properties, and these hardness properties, if desirable, should not be sacrificed if the pixel size is decreased in an effort to increase resolution.
Designing the imaging sensor to have desirable radiation hardness properties may also cause unforeseen and unappreciated problems. For example, certain device features are more susceptible to radiation effects if their overall dimensions are larger, so there is a natural desire to decrease the dimensions of these device features. The decrease in device dimensions may, e.g., drive the sensor thickness to dimensions that are difficult to fabricate using conventional fabrication methods.
Thus, those skilled in the art desire new imaging sensor designs that have improved resolution with desirable noise and radiation hardness properties. Those skilled in the art also desire fabrication methods that are capable of economically making new imaging sensors with desirable properties by achieving acceptable yield levels. Those skilled in the art further desire designs and methods that are particularly applicable to PIN imaging sensors.